1. Field of Invention
The invention generally relates to a projection lens, and more particularly, to a fixed focal length lens.
2. Background of the Invention
In general, a long projection distance is required if a projector needs to project an image onto a large screen. Contrarily, a special wide-angle lens is required to shorten the distance from the screen to the projector if the image is to be projected onto the large screen from a short projection distance. That is, the wide-angle lens may effectively reduce the distance from the screen to the projector and project a relatively large image. However, the aberration issue derived from the wide-angle lens is one of the obstacles faced by designers.
There are a number of solutions to the aberration issue, e.g., through the use of plural aspheric lenses, the increase in the total length of the lens, the use of a number of lenses, and so on. For instance, in U.S. Pat. No. 6,621,645, at least one molded glass is applied, thus increasing the costs. Besides, the distortion value disclosed in said patent is greater than ±1%. In U.S. Pat. No. 6,560,041, at least three aspheric lenses are employed, which raises the manufacturing costs and the assembly difficulty. If only a few aspheric lenses are to be used, and it is intended to effectively correct the aberration, the total length of the lenses may be increased, and the volume of the projection system may be enlarged. For instance, in U.S. Pat. Nos. 6,999,247 and 6,542,316, the total length of the lenses is greater than 150 mm. It is also likely to correct the aberration by utilizing a large number of lenses. For instance, in U.S. Pat. Nos. 6,621,645 and 7,184,219, at least 14 lenses are applied in the projection lens for aberration correction. Besides, in Taiwan patent no. 1247915 and U.S. Pat. No. 7,952,817, at least 13 lenses are applied in the projection lens for aberration correction.
In U.S. Pat. No. 7,423,819, the fixed-focus lens includes a first lens group, a second lens group, and a third lens group that are sequentially arranged from an object side to an image side, and the third lens group includes triple cemented lenses. U.S. Pat. No. 7,859,770 satisfies the following: if F/H>0.627, the wide-angle effect may be achieved, and the aberration reaches the minimum value and satisfies 0.5<|F1/F|<1.7 and 1.9<|F2/F|<3.1, where F refers to a focal length of the lens, F1 refers to a focal length of the first lens group, and F2 refers to a focal length of the second lens group. Lens-related technologies have also been disclosed in numerous patents, such as U.S. Pat. Nos. 7,126,767, 7,123,426, and 7,173,777.
In view of the above, how to design a lens with low manufacturing costs and favorable imaging quality has become one of the research topics to people having ordinary skill in the pertinent field.